pre-ignition on 425

DAN76 · October 10th, 2008, 06:51 AM

not sure if this is common with american V8's but my engine continues to run for a few seconds after the ignition is turned off!!

There's no banging or pinking noises so i'm assuming it's just carbon build up in the fuel system???

Has anyone else experienced this?

BILL DEMMER · October 10th, 2008, 07:50 AM

it is more common with older/high-compression motors. carbon build-up can also contribute to this. first, i would check your idle speed. my '69 350 used to do this when the idle speed was too high and the previous owner had defeated the anti-diesel solenoid's function. iirc, your idle speed s/b 550-575 rpm in park, assuming your switch-pitch th400 is functional. if you're not getting any pinging/knocking under moderate load, i would say that any carbon build-up is minimal and not the cause of the dieseling.

bill

DAN76 · October 12th, 2008, 04:07 AM

it varies.
sometimes it doesn't happen at all but every now and then it will run-on for a couple of seconds, emitting a nasty smell and sometimes a small amount of smoke from under the hood.

I've added some Reddex to the tank which seemed to work for a while but started to happen again soon after.

I will adjust the idle speed and see what happens.

I have also noticed the my engine stays very hot for a long time after running. I was working on the fuel pump the other day and the car had been left standing for 1 hour after a 10 minute drive. The engine was still too hot to touch after this time - is that normal?

zed · October 12th, 2008, 07:53 AM

A habit I developed from having higher compresion ratio motors was to shut the engine off while idling in Drive. Then throwing it into Park. The lower idle speed helps, assuming it an auto trans.

DAN76 · October 13th, 2008, 02:31 AM

Quote:

Originally Posted by zed

A habit I developed from having higher compresion ratio motors was to shut the engine off while idling in Drive. Then throwing it into Park. The lower idle speed helps, assuming it an auto trans.

This is the exact method i have now adopted!

The tick over is definitely a tad fast, and since I treated the car to a full tank of 97 RON fuel and a dose of Redex, it now it ticks over even faster!
So have been switching off in gear and then setting to park. The run-on hasn't happened since doing this!

zed · October 13th, 2008, 07:57 AM

Glad to hear.

October 10th, 2008, 06:51 AM	#1 (permalink)
DAN76 Englishman Join Date: Oct 2008 Location: London UK Posts: 184	pre-ignition on 425 not sure if this is common with american V8's but my engine continues to run for a few seconds after the ignition is turned off!! There's no banging or pinking noises so i'm assuming it's just carbon build up in the fuel system??? Has anyone else experienced this? __________________ 1965 Delta 88 Hard Top

October 10th, 2008, 07:50 AM	#2 (permalink)
BILL DEMMER Just the facts... Join Date: May 2008 Location: THE GREAT WIDE-OPEN Posts: 208	it is more common with older/high-compression motors. carbon build-up can also contribute to this. first, i would check your idle speed. my '69 350 used to do this when the idle speed was too high and the previous owner had defeated the anti-diesel solenoid's function. iirc, your idle speed s/b 550-575 rpm in park, assuming your switch-pitch th400 is functional. if you're not getting any pinging/knocking under moderate load, i would say that any carbon build-up is minimal and not the cause of the dieseling. bill __________________ '67 CUTLASS 4-DR. W/425 & S.P. THM400 (NAMED "ERNIE") '83 H/O #1339(STILL IN PIECES) '85 PONTIAC FIERO SE V-6 4-SPD. BOUGHT NEW '92 CHEBBY DIS-ASTRO (GAS PIG) '97 GYRO METRO $HYTBOX (41.42 MPG) Last edited by BILL DEMMER; October 10th, 2008 at 07:53 AM..

October 12th, 2008, 04:07 AM	#3 (permalink)
DAN76 Englishman Join Date: Oct 2008 Location: London UK Posts: 184	it varies. sometimes it doesn't happen at all but every now and then it will run-on for a couple of seconds, emitting a nasty smell and sometimes a small amount of smoke from under the hood. I've added some Reddex to the tank which seemed to work for a while but started to happen again soon after. I will adjust the idle speed and see what happens. I have also noticed the my engine stays very hot for a long time after running. I was working on the fuel pump the other day and the car had been left standing for 1 hour after a 10 minute drive. The engine was still too hot to touch after this time - is that normal? __________________ 1965 Delta 88 Hard Top

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October 12th, 2008, 07:53 AM	#4 (permalink)
zed Registered User Join Date: Oct 2008 Location: Saskatchewan Prairies Baby! Posts: 142	A habit I developed from having higher compresion ratio motors was to shut the engine off while idling in Drive. Then throwing it into Park. The lower idle speed helps, assuming it an auto trans.

October 13th, 2008, 07:57 AM	#6 (permalink)
zed Registered User Join Date: Oct 2008 Location: Saskatchewan Prairies Baby! Posts: 142	Glad to hear.